1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses such as printers that are devised so as to prevent smearing of the exposure unit due to natural dropping of the developer and spattering of the developer, and particularly relates to measures for preventing soiling on the optical path of light that is exposed from exposure units.
2. Description of the Related Art
Conventionally, the layout of the paper-supply unit, printing unit, paper-discharge unit, and optical units (there are two types, an optical unit for reading manuscripts for printing and an optical unit for exposure that writes the image information that has been read to an electrostatic latent image carrier) that constitute an image forming apparatus has been determined with consideration to such factors as compactness of the apparatus, ease of operation, and ease of designing the apparatus.
Normally, the development unit that accommodates developer is positioned on the side of the photosensitive body, and the optical unit for exposure (in recent years, reading heads such as LSU, EL, and LED reading heads are used) is positioned above or diagonally above the photosensitive body, with many forms of exposure being used.
On the other hand, with color image forming apparatuses that strive for device compactness, a photosensitive body and a development unit are required for each color, but since the fixing device and sheet carry path are not required for each color, development has advanced of color image forming apparatuses known as “tandem technique,” in which a color image is obtained in one pass of the paper. There are two styles of tandem technique, one being a style in which image information that is made as a development on the photosensitive body for each color is transferred directly to paper, and the other being a style in which image information that is made as a development on the photosensitive body for each color is temporarily superimposed on an intermediate medium (an intermediate transfer body), then transferred in a single transfer step to a sheet of paper. In this case, photosensitive bodies and development units for yellow, magenta, and cyan colors are of course respectively positioned in the color image forming apparatus, and naturally a photosensitive body and development unit for black, which is a high proportion of a user's printing, are also provided.
Here, in the latter case of the style using an intermediate transfer medium (hereinafter, an “endless belt”), the layout position of the photosensitive bodies arranged at the periphery of the endless belt is one determinant of the printing speed (fast copy speed) of the apparatus. That is to say, it is a necessary condition in order to achieve improved printing speed to respectively arrange a photosensitive body and development unit for the colors yellow, magenta, and cyan in order from a position distant from the transfer unit that transfers these colors together onto a sheet of paper, and to arrange a photosensitive body and development unit for black, which is the most printed color, in the closet position.
In regard to this, many apparatuses of the built-up format, which is also referred to as front access style, have been developed in recent years to reduce the area occupied by image forming apparatuses. That is to say, many apparatuses have been released in which a storage/supply unit for printing paper is arranged at the lowermost area of the apparatus, with the printing unit arranged above this and a scanner unit that reads the image information of the manuscript arranged at the highest position of the apparatus, while discharge of the printing paper is carried out at a side of the apparatus or an upper area of the printing unit with the scanner unit in a lower area.
In such apparatuses, in cases in which a technique is applied using an endless belt with the tandem technique, the carry direction of the paper is from bottom to top and the transfer unit is basically conceived as rotating in the same direction as the paper carry direction and, moreover, in considering the optimal layout positions of the paper transfer unit and the photosensitive body and development unit for each color, as well as improvements in printing speed, the layout from the lowermost area of the apparatus goes from the paper storage/supply unit, the optical unit for exposing the photosensitive body, the photosensitive bodies and the development units, and then the endless belt.
In this way, when positioning the development range (normally called “development nip”) formed by the photosensitive body and the development sleeve with the development unit positioned above the optical unit for exposure, residual developer at the development nip area is caused by such factors as the flow of air produced by the peripheral speed of the photosensitive body and the development sleeve, or the loss of an electric field effect due to an electric field of the development sleeve, to be subjected to natural dropping by gravity or spattering within the machine.
In this way, the optical unit for exposure positioned below the development unit may be soiled by developer, which tends to cause reduced printing quality by disturbing the written image or damaging the polygon mirrors that rotate at high speed, as well as smearing the fθ lens and the plurality of mirrors.
From this point, regarding the relationship in which the optical unit for exposure is positioned below the photosensitive body and the development unit, a structure is necessary that shields this area from the outside in order to always keep clean the optical unit for exposure.
However, since the optical path from the optical unit for exposure cannot be blocked in order to write image information to the photosensitive body, it is necessary to leave an aperture portion at the portion where the optical path is incident, and this results in ingression of developer from the aperture portion into the optical unit for exposure.
Accordingly, apparatuses have been proposed (see JP H04-323669A for example) in which an openable-closeable shutter is arranged at the portion (aperture portion) where the optical path crosses from the optical unit for exposure, and prevents soiling caused by the ingression of developer from the aperture portion, thus keeping the optical unit for exposure always clean.
Furthermore, there are apparatuses of other techniques in which glass is arranged at the portion where the optical path of the optical unit crosses such that soiling caused by the ingression of developer from the aperture portion is prevented, thus keeping the optical unit for exposure always clean.
However, with an apparatus such as that mentioned above, which is provided with an openable-closeable shutter at the portion (aperture portion) where the optical path crosses from the optical unit such as the optical unit for exposure, there is uncertainty as to whether or not the shutter mechanism can properly fulfill its function in regard to the operation of the apparatus. That is to say, a greater portion of the dropping and spattering of the developer is produced during the printing process and at that time the shutter is required to stay open during exposure so that the exposure unit can write image information to the image carrier such as the photosensitive body. For this reason, when the shutter is in an open state, it is unable to block the dropped/spattered developer and is unable to prevent soiling by developer inside the exposure unit such that the shutter function cannot be considered to be sufficiently effective. In short, the shutter is for preventing the dropping of the developer during movement of the apparatus or during replacement operations of the developer or the image carrier or the like, and is not able to prevent soiling caused by ingression of the developer into the exposure unit at the time of using exposure to write image information onto the image carrier.
In contrast to this, with an apparatus such as that described above in which glass is arranged at a portion (aperture portion) where the optical path crosses from the exposure unit, this is nothing more than simply the arrangement of transparent glass on the optical path of the exposed light, and even if the glass is cleaned periodically, the risk of dropped/spattered developer accumulating on the glass is posed during the cleaning interval from after cleaning until the next cleaning, and if the optical path is soiled by developer, image information cannot be accurately written to the image carrier using exposure.
The present invention has been devised in consideration of these issues, and it is an object thereof to provide an image forming apparatus that can reliably prevent ingression of developer to the exposure unit when writing onto an image carrier by exposing image information and is capable of accurately writing on the image carrier by exposing image information by preventing soiling on the optical path of the exposed light by developer.